The present invention is directed to a technique for heat conditioning glass substrates, which are subsequently surface treated by vacuum surface treatment. The invention departs from requirements which are encountered when large glass substrates for display panel production from 300 cm2 up to 1 m2 and more have to be heat conditioned.
For subsequent vacuum coating surfaces of glass substrates in the flat panel industry, as by aluminum sputtering, water molecules must previously have been removed from the surface of the substrate. This is customarily performed by heating which additionally degases and cleans the surfaces from further surface contaminations.
To perform such heat conditioning it is known to convey a multitude of substrates into an oven which is heated e.g. up to 250xc2x0 C. The oven is operated at vacuum atmosphere. Thereby, considerable time lapses, until the substrates reach the final desired temperature. So as not to deteriorate the high throughput of the overall vacuum processing plants by such low-speed process, a multitude of substrates must be heat conditioned simultaneously, whereas other processes of the overall processing plant are performed in single substrate mode. Thus, one can say the single processed substrates are-collected for heating. If other processes of the plant operate in batch mode, each batch then representing one workpiece, the described approach necessitates accordingly multiple batch heating.
It is a prime object of the present invention to provide for a heat conditioning process and heat conditioning chamber by which an improved heating rate of the glass substrate is achieved compared with prior art attempts, so that, especially in a cluster-type processing vacuum plant, heat-conditioning may be performed by single substrate operation without limiting overall throughput of the plant. Thus, the inventive technique shall fully comply in time consumption with single workpiece treatment, be it single substrate treatment or single batch treatment, where xe2x80x9ca batchxe2x80x9d is considered as xe2x80x9cone workpiecexe2x80x9d with respect to heat treatment. This object is realised by the inventive process for heat conditioning at least one glass substrate for subsequent surface treatment by at least one vacuum surface treatment process, as for subsequent vacuum coating, which inventive process comprises the steps of
introducing said substrate into a chamber;
having said chamber evacuated before the introducing or evacuating said chamber after the introducing;
predetermining the spectral absorption characteristics of the substrate in the infrared spectral band and including its lower slope, where absorption rises with increasing wavelength;
selecting at least one lamp with a radiation spectrum band overlapping said absorption spectrum of said substrate at least along a predominant part of the slope and/or towards longer wavelengths;
exposing the substrate in the evacuated chamber to radiation from the lamp directly via the evacuated atmosphere of said chamber.
The infrared spectral band is here defined by
500 nmxe2x89xa6xcexxe2x89xa610000,
wherein xcex is the wavelength of light.
In a preferred mode of the inventive process a lamp is selected with a radiation peak at a wavelength xcexr
1500 nmxe2x89xa6xcexr,
thereby there is preferred
1500 nmxe2x89xa6xcexrxe2x89xa66000 nm,
thereby especially
2000 nmxe2x89xa6xcexrxe2x89xa66000 nm,
and especially preferred
2500 nmxe2x89xa6xcexrxe2x89xa64500 nm.
It is further preferably proposed to reflect radiation transmitted through the substrate back towards the workpiece, so as to minimise thermal loss.
Also it would be possible to provide therefor a rigid structure with a reflecting surface, in a further preferred form of realisation reflecting is performed by means of a foil-like or mise thermal lose. The material of the reflecting surface is thereby preferably selected so as to reflect light in the radiation spectral band of the lamp to at least 50%, even to at least 80%, thereby absorbing minimal energy. As material of a reflecting surface preferably aluminum is selected. In combination with such Aluminum xcexr should be selected above 1500 nm to minimise absorption of lamp radiation and thus to minimise system inertia.
Especially with Aluminum as material of the reflecting surface more than 99% of radiation in the near infrared and infrared spectral band may be reflected and approx. 90% of radiation in the visible spectral range.
In spite of the fact that double-side direct exposure to respective lamps is possible in a preferred mode, heating the substrate is performed by direct exposure to the lamp radiation from one side and reflecting radiation from the other side back to the substrate, which has the advantage that the substrate is thermically loaded substantially equally and more efficiently from both sides, which prevents thermal stress warping and deterioration of the substrate by inhomogeneous heat loading.
Thereby it must be emphasised that thermal gradients in the substrate would lead to different magnitude expansion within the substrate, thereby leading to bending of the glass up to breakage. This is clearly to be avoided.
To further prevent overheating of the reflector arrangement, in a further preferred mode the reflector arrangement, and especially a foil-like or sheet-like reflector arrangement, is cooled from its side unexposed to the radiation from the lamp. This is preferably performed via a rigid chamber wall distant from and adjacent to the reflector arrangement and fluid-cooling the rigid wall, which is preferably made of stainless steel. This cooling effect is preferably further improved by providing a black-body radiation coating on that side of the reflector arrangement which is not exposed to the radiation of the lamp.
So as to improve heating homogeneity along the substrate, especially along a large substrate, it is proposed to provide more than one lamp and to preferably provide such lamps at respectively selected different mutual distances, which distances are preferably adjustable.
Up to now the present invention was optimised with respect to heating efficiency. Nevertheless, cooling efficiency may be as important as heating efficiency in view of the overall heat conditioning treatment cycles. To optimise cooling efficiency the heat transfer mechanism isxe2x80x94for coolingxe2x80x94switched to completely different physics, namely from radiation heating to conductance cooling. In a preferred mode of operating in the cooling cycle, a heat conducting gas, as preferably a noble gas, as especially Helium, is introduced into the treatment chamber. Especially as the outer wall of the chamber is cooled, heat conductance leads to rapid decrease of substrate temperature.
With respect to evacuating the treatment chamber it is recommendedxe2x80x94for heating operationxe2x80x94to pump it down to a vacuum where heat conductance practically ceases. This to avoid thermal losses to the surrounding and to rise heating efficiency. Further, the lamp is preferably positioned beneath the substrate to prevent particle contamination of the substrate. The heating rate may be controlled by means of a negative feedback control loop, thereby using preferably a pyrometer sensor directed towards the substrate to detect its actual thermal state to be compared with a desired thermal state value. The resulting comparison result, as a control difference, acts on the lamp control to adjust its power and/or thereby especially to shift its radiation spectrum, and/or on a gas inlet control valve so as to control thermal conductivity and thus thermal loss and/or on the control of an evacuating pump. It has to be noted that by controlling the electrical supply of the lamps their radiation spectrum may be shifted, which again leads to adjustments of the thermal state of the substrate.
We recommend the use of so-called xe2x80x9cblack-type lampsxe2x80x9d sold by the firm USHIO Inc. or of carbon-radiator lamps as available from the firm HERAEUS Inc.
Thus, more generically halogen lamps are preferred with a black coating on the glass bulb. Thereby transformation of energy in the visible spectral range or in the near infrared range is transformed in energy in the infrared spectral band.
With respect to selection of aluminum as reflecting material it has to be noted that it has an absorption maximum at around 800 nm, but has up to 99% of reflectance beyond 2000 nm, which is optimum, as glass begins to absorb at around 2500 nm. The xe2x80x9cblack-typexe2x80x9d lamps mentioned above have a maximum of radiation spectral band at about 4000 nm, depending on their surface temperature, the carbon radiator lamps at about 2000 nm well matched with the absorption characteristics of glass used for flat panels au of Corning glass. The carbon radiator lamp has the further advantage that the response time is shorter and its behaviour, with respect to low particle generation, is better.
Further, the technique of cooling the chamber walls leads to a thermal stable reference for the overall heating system control.
With the inventive process and accordingly with the inventive chamber it was possible to repeatedly and reliably heat Borosilicat glass substrates with a surface of at least 300 cm2 up to 1 m2 from room temperature to 200xc2x0 C. in 50 sec. without significant temperature overshoot and with an excellent distribution of temperature along the substrate surface. Temperature overshoot was at most 10%, normally quite lower.
As water and oxygen, which are mainly present at the surface of unconditioned substrates, start to desorb at about 130xc2x0 C., with the high heating rate mentioned, it is possible to degas the surfaces at least to a very high extent in a very short time, which is rather shorter than previously or subsequently performed treatment processes in a cluster-type flat panel producing plant.
Back to the inventive chamber, to fulfil the above mentioned object it comprises a rigid outer wall with at least one input/output lock or at least one input and at least one output lock, a workpiece holder arrangement within the chamber, a vacuum pumping arrangement operationally connected to the chamber, at least one lamp opposite said workpiece holder arrangement and freely accessible from said workpiece holder, said lamp having a radiation spectrum band overlapping the absorption spectrum band of glass at least along a predominant part of the absorption slope of said band of said glass where absorption rises with increasing wavelength and/or at longer wavelengths.
Further preferred features of the invention will now be described by examples with the help of figures. Therein, it in shown: